The invention relates to pressurized water nuclear reactors and is for use in the same environment as U.S. patent application Ser. No. 07/582,589, filed substantially concurrently with the present application, by the same inventor, for "Bottom Nozzle to Lower Grid Attachment". More specifically, the device relates to the lower core plate to fuel assembly interface.
The invention is an improved multistage design created more evenly to distribute the flow within and above the lower end fitting to decrease the jet flow impingement on fuel rods. This improves reactor performance by improving the performance of diffusing lower core plate flow jets. The desired result sought and achieved is a decrease of flow induced vibration of the fuel rods and stiffening and lightening of the lower end fitting. In the past, flow induced vibration of fuel rods in their lower spans has caused high fuel rod wear rates. This has resulted in the use of a stronger grid and more massive support system in the lower section of the fuel assemblies. The current and prior art practice of using inconel lower grids, with their high neutron capture, is to achieve these required supporting forces. The need for the improved lower end fitting thus also results from the desire for vibration elimination which would allow use of a low parasitic material such as Zircaloy 4. Use of an integral wear-reduction-shield of the type taught in U.S. Pat. No. 4,888,149 issued to the inventor of the present invention also provides strength to the new lower end fitting.